The invention relates to a vane intended for a headbox of a paper machine or equivalent. The invention also relates to a method for ensuring the straightness of a vane intended for a headbox.
By the paper machine or equivalent is meant paper, board and soft tissue machines.
Vanes are used in the slice channel of the headbox for damping large-scale turbulence and for converting it into small-scale turbulence to thereby prevent the formation of fiber bundles and improve the quality of the paper to be manufactured. Vanes are also used in multi-layer headboxes for separating the different layers of flow from one another.
Vanes can be manufactured of metal, plastic or a composite composed of a binder and fibers embedded in the binder. They can be of equal thickness in cross-section or thinning in a wedge-like manner towards a trailing edge. The vanes are attached by their leading edge, as viewed in the flow direction, to an attachment groove provided in the turbulence generator of the headbox, so that their trailing edge is able to float freely with the flow in the slice channel of the headbox. Between the vanes there are tapering flow channels, in which the velocity of the pulp suspension accelerates towards the slice opening of the headbox. With respect to the uniformity and undisturbed state of the flow discharging out of the slice opening it is essential that the vanes placed in the slice channel of the headbox are straight and parallel.
It has been found that a vane made of plastic or a composite absorbs some water in the moisture and temperature conditions of the headbox, so that the dimensions of the vane may change as a result of swelling. When a new wedge-shaped vane is fitted in the headbox, its thin trailing edge reaches an equilibrium moisture content much more quickly than the thick leading edge. This readily leads to the fact that the vane warps and undulation is produced in it, which adversely affects the quality of the paper being manufactured.
The temperature and moisture conditions during the manufacture, transport and storage of vanes are generally different from the operating conditions in the headbox. Vanes intended for the headbox have to be stored in varying conditions for varying periods of time before they are put into use. For the reasons set out above, vanes whose moisture content is different from the equilibrium moisture content of the vane during use in the headbox must be fitted in the paper machine. It may take several weeks to reach a state of equilibrium when vanes are put into use, and during that time the quality of paper readily suffers from the uneven shape of the vanes.